Cartridge fuse having blown fuse indicator

ABSTRACT

A cartridge fuse is provided with a blown fuse indicator springbiased to an indicating position and normally held in its nonindicating position by means of a restraining wire. The fuse includes effective mechanical means rather than a solder joint for precluding undesired slippage of the restraining wire. To achieve this end the tubular casing of the fuse is provided with a substantially transverse opening, and the restraining wire is threaded through said opening and forms a loop firmly anchoring the end of the restraining wire formed into the loop and clamped by the lateral surface of a ferrule against the outer surface of the casing of the fuse.

United States Patent [191 Belcher 51March 20, 1973 CARTRIDGE FUSE HAVING BLOWN FUSE INDICATOR [75] Inventor: Richard A. Belcher, Hampton Falls,

[73] Assignee: The Chase-Shawmut Company,

Newburyport, Mass.

221 Filed: March 29,1972

[2]] Appl. No.: 239,090

[52] US. Cl ..337/241, 337/244 [51 Int. Cl. ..H01h 85/30 [58] Field of Search ..337/24l, 244, 248, 265, 267

[56] References Cited UNITED STATES PATENTS 2,274,059 2/l942 Goodwin ..337/265 X 1,081,2l3 l2/l9l3 Conant ..337/244 3,523,264 8/1970 Feenan et al. ..337/244 X Hitchcock .337/244 X Primary Examiner-Bernard A. Gilheany Assistant ExaminerF. E. Bell Att0rneyErwin Salzer 57 ABSTRACT A cartridge fuse is provided with a blown fuse indicator spring-biased to an indicating position and normally held in its non-indicating position by means of a restraining wire. The fuse includes effective mechanical means rather than a solder joint for precluding undesired slippage of the restraining wire. To achieve this end the tubular casing of the fuse is provided with a substantially transverse opening, and the restraining wire is threaded through said opening and forms a loop firmly anchoring the end of the restraining wire formed into the loop and clamped by the lateral surface of a ferrule against the outer surface of the casing of the fuse.

5 Claims, 9 Drawing Figures PATENIEDMARZUIQ sum 1 BF 2 FIG 2 PRIOR ART FIGJ. PRIOR ART r VVI J FIG.4.

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CARTRIDGE FUSE HAVING BLOWN FUSE INDICATOR BACKGROUND OF THE INVENTION wire melts upon blowing of the fuse, thus allowing the m blown fuse indicator to be moved to its indicating position under the action of its biasing spring.

There are instances where the spring-biased blown fuse indicator becomes operative and suggests that the particular fuse has blown, though the fuse has not blown in fact. Such failures of blown fuse indicators are a source of great annoyance, result in downtime with incident decrease in productivity, and cause serious increase in the cost of production since a fuse whose blown fuse indicator is defective must be discarded though the fuse proper is intact and fully operative.

One of the reasons underlying failure of springbiased blown fuse indicators is slippage of the restraining wire under the action of its biasing spring. Such slippage may occur at a very slow rate. A fuse may have successfully past rigid quality controls, but after several months of shelf-life the restraining wire may have gradually slipped sufficiently to allow the blown fuse indicator to move almost imperceptivcly from its nonindicating position to its indicating position. The danger of such failures increases the larger the force of the spring used to bias the blown fuse indicator. Where the blown fuse indicator is intended to perform some mechanical work, in addition to indicate visually that a fuse has blown, the spring bias of the blown fuse indicator must be relatively large, and consequently the danger of slippage of the restraining wire increases in proportion to the increase of the spring bias of the blown fuse indicator.

Another cause for unintentional operation of a spring-biased blown fuse indicator is breaking of the restraining wire thereof. The danger of breaking of the restraining wire is particularly large in instances where the restraining wire is bent around a sharp edge, or

corner, and subjected at this point to a high specific pressure.

There are many cartridge fuses including springbiased blown fuse indicators which are not subject to the above drawbacks and limitations, but in others these drawbacks and limitations are very serious. This will be explained below more in detail.

Virtually all fuses which rely only or mainly on a solder joint for anchoring one end of the restraining wire of the blown fuse indicator are subject to gradual slippage of the restraining wire and gradual movement of the blown fuse indicator to its indicating position. This limitation is due to the fact that the solder joint is subject to cold flow.

Particular difficulties are encountered in manufacturing ferrule type low-voltage fuses having a blown fuse indicator adjacent one end surface of one of their ferrules. These difficulties are due to the fact that the end of the restraining wire for the blown fuse indicator remote from the latter must be firmly affixed to the fuse structure in such a way as to preclude any damage to that relatively thin and fragile wire, and in such a way as to preclude the slightest slippage, ore creepage, of

the wire under the action of the spring bias of the blown fuse indicator. This problem has apparently not found any solution as yet.

The above problem is peculiar to fuses whose terminals are formed by ferrules rather than by plugs, plates, or other types of terminals. U.S. Pat. No. 2,834,852 to K.W.Swain et a]., May 13, 1958 for PLUG-IN TYPE POWER FUSES shows how a restraining wire for a blown fuse indicator can readily and firmly be attached to a plug terminal of a fuse. This way of attachment of a restraining wire is, however, generally not applicable to ferrule type fuses, particularly if their current rating is relatively small.

It is common practice in the' design of cartridge fuses which have ferrule terminals to bend the ends of fusible ribbon elements around the rims formed by the axially outer ends of the casing upon the outer surface of the casing, and to clamp the portions of the fusible elements positioned on the outer surface of the casing against said outer surface, thus firmly maintaining the fusible elements in the proper position thereof. A fuse structure of this character is disclosed in U.S. Pat. No. 3,294,937 to FJ. Kozacka, Dec. 27, 1966 for TIME LAG DUAL ELEMENT FUSE HAVING MEANS FOR PRECLUDING ARC INITIATION ADJACENT TERMINALS.

Attempts have been made to anchor ends of restraining wires for blown fuse indicators in the same fashion as ends of ribbon fuse links are shown in the above Kozacka patent to be anchored. These attempts resulted in failures because the problem of anchoring thin relatively hard and fragile wires under tension of spring means is entirely different from that of affixing wide, relatively soft and pliable ribbon fuse links which are under no significant tension. The pressure of the axially inner end surface of a ferrule upon that portion of a ribbon fuse link which is clamped between said surface and the rim of a fuse casing has no tendency to damage the ribbon fuse link. Such pressure, however, tends to damage a restraining wire the end of which is clamped between the axially inner end surface of a ferrule and the rim portion of a fuse casing. A ribbon fuse link of silver or copper bent around the rim portion of a casing to the outer surface thereof tends to hug more or less the outer surface of the casing. Hence a ferrule can readily be mounted on the casing covering the portion of the ribbon fuse link situated on the outer surface of the casing. A restraining wire for a blown fuse indicator is generally made of steel having a relatively high modulus of elasticity. As a result, the end of a restraining wire bent around the rim of a casing tends to spring back to its initial position and may then project at such an angle from the rim of the casing that there is a strong tendency for the portion of the wire projecting beyond the rim of the casing to be sheared-off by the ferrule when the latter is being mounted upon the casing.

It is the principal object of this invention to provide simple and inexpensive ferrule type fuses having spring biased blown fuse indicators including restraining wires which wires are free from the danger of slippage, firmly affixed to the casing and not subject to the serious drawbacks normally resulting from the application of the above clamping method for ribbon-fuse links to restraining wires of blown fuse indicators which simply means effectively avoiding the above drawbacks and limitations.

SUMMARY OF THE INVENTION Fuses embodying this invention are provided with means precluding slippage of the restraining wire or damage to the latter on account of excessive specific pressures. Slippage of the wire is avoided by providing the ends of the casing of the fuse remote from the spring-biased blown fuse indicator with a substantially transverse hole or aperture and forming the restraining wire of the blown fuse indicator into a loop by threading the restraining wire through the hole or aperture in the casing. One portion of the aforementioned loop is situated on the outer surface of the casing and clamped by a ferrule against it. The invention is particularly intended to be applied to cartridge fuses having a pair of ferrules or terminal caps on each end of the casing thereof.

BRIEF DESCRIPTION OF DRAWINGS FIGS. 1 and 2 are diagrammatic representations of what is likely to happen when the prior art ribbon fuse link anchoring method is applied to restraining wires of blown fuse indicators and these figures show in longitudinal section two stages of the process of assembly;

FIGS. 3 and 4 are diagrammatic representations showing how the restraining wire of a blown fuse indicator is anchored according to the present invention, these figures showing in longitudinal section two stages of the process of assembly;

FIGS. 5,6 and 6d are diagrammatic representations of a modification of the structure and the process steps illustrated in FIGS. 3 and 4;

FIG. 7 is a side elevation of a fuse embodying the present invention; and

FIG. 8 is a section along VIII-VIII of FIG. 7

DESCRIPTION OF PREFERRED EMBODIMENTS In FIGS. 1 and 2 reference character 1 has been applied to indicate a tubular fuse casing of insulating material, reference character 2 has been applied to indicate a ferrule intended to be mounted on casing l, and reference character 3 has been applied to indicate a restraining wire of steel having a relatively high modulus of elasticity. All other parts of the fuse including its current-carrying fusible element, its pulverulent arc-quenching filler, and its spring-biased blown fuse indicator have been deleted in FIGS. 1 and 2 since these parts of the fuse are not relevant to what FIGS. 1 and 2 are supposed to illustrate. As shown in FIG. 1, restraining wire 3 is bent over the rim formed by the axially outer end of easing 1 into engagement with the outer surface of casing 1. Originally restraining wire 3 is bent to form a hair-pin loop or U-shaped loop as indicated by a dotted line to the right of FIG. 1. Due to the resiliency of steel wire 3 its end is likely to spring back more or less to the position shown in solid lines in FIG. 1. FIG. 2 shows the instant when the rim of cap or ferrule 2 has engaged the end of restraining wire 3 projecting from casing l and severed that end from the rest of restraining wire 3 by a shearing action. Now the fuse whose manufacturing process was about to be completed has become a total loss.

Assuming that nobody noticed that the end of restraining wire 3 was chopped off by the cutting edge or cutting rim of ferrule 2; in that event the blown fuse indicator moves sooner or later from its non-indicating position to its indicating position, though the fusible element (not shown) of the fuse is still intact and though the fuse may never have carried current.

If the restraining wire 3 is not cut-off by ferrule 2, as assumed above, but bent down by ferrule 2 on the outer surface of easing 1, the chances are that the high pres I is being stored, or after it has been put to service. Such I belated breaking of restraining wires is brought about by the initial damage to them by excessive pressure concentrated on a single point or short length, coupled with the constant pulling action of the biasing spring of the blown fuse indicator. This instance of failure of the blown fuse indicator is thus due to metal fatigue.

Assuming ferrule 2 is provided with a hole, the end of restraining wire 3 is threaded through the latter and soldered to the outside of ferrule 2 by a solder joint. Such a solder joint is subjected to cold flow, even if the spring action upon, and tension of, wire 3 is of relatively limited magnitude. The solder joint thus gradually releases restraining wire 3 so that the blown fuse in dicator creeps gradually and almost imperceptively from its non-indicating position to its indicating position.

In FIGS. 3 and 4 reference character 1' has been applied to indicate a tubular fuse casing of insulating material, reference character 2' has been applied to indicate a ferrule and reference character 3' has been applied to indicate a restraining wire for a spring-biased blown fuse indicator. The action of the spring has been indicated by an arrow. All other parts of the fuse such as its fusible ribbon element, its pulverulent arcquenching filler, its blown fuse indicator, etc. have been deleted in FIGS. 3 and 4 for the reasons stated above for their deletion in FIGS. 1 and 2. As shown in FIGS. 3 and 4 casing l is provided with a radial slot 1a in the end of casing l remotefrom the blown fuse indicator. Slot la extends in a direction longitudinally of casing 1. The radially inner end lb of the bottom of slot la is positioned more remotely from the transverse center plane A-A(see FIG. 8) of casing 1' than the radially outer end 10 of the bottom of said slot la. As a result, the bottom of slot la forms an oblique surface converging toward ferrule 2. The upper end of restraining wire 3', i.e., the end thereof remote from the blown fuse indicator(not shown), is inserted into slot 1a and projects beyond the radially outer end 1c of the bottom of slot la. This end of restraining wire 3 is intended to be clamped by ferrule 2' against the outer surface 1d of casing l.

FIG. 4 shows an instant in the process of mounting ferrule 2' upon the upper end of fuse casing 1'. The edge of the rim of casing l spreads ferrule 2' to assume more or less the shape of a frustrum of a cone. At the moment the inner surface of ferrule 2' engages the portion of restraining wire 3' projecting beyond the outer surface 1d of casing 1', the inner diameter of ferrule 2' has been enlarged and the inner surface of ferrule 2' encloses such an angle with the outwardly projection portion of wire 3 that ferrule 2 bends the portion of wire 3' gently and firmly into engagement with the outer surface 1d of casing 1'. The restraining wire 3' engages theouter surface 1d of casing 1' only in a region thereof spaced from the adjacent rim le of casing 1'. When ferrule 2 is completely mounted upon casing l, restraining wire 3' is clamped by the lateral surface of ferrule 2' against casing 1. The length of the clamped portion of restraining wire 3 is sufficient to firmly hold it in position against the pull of the biasing spring of the blown fuse indicator. No increment of the restraining wire 3' is clamped between the rim 1e of casing 1' and the' inner end surface of ferrule 2. When ferrule 2 is fully mounted upon casing l a minimum spacing s is=left between restraining wire 3' and the inner end surface of ferrule 2. Thus pressure upon restraining wire 3' where such pressure may, or tends to be, excessive is entirely avoided.

The structure of FIGS. 5 and 6. is very similar to that shown in FIGS. 3 and 4 but designed to anchor restraining wires which are subjected to a relatively strong pull which occurs when it is intended to use a blown fuse indicator pin to perform some mechanical duty, e.g., to trip a circuit breaker.

In FIGS. 5 and 6 reference character 1" has been applied to indicate a tubular fuse casing of electric insulating material, reference character 2" has been applied to indicate a ferrule for closing the upper end of easing 1" and reference character 3" has been applied to indicate a restraining wire for a spring-biased blown fuse indicator. Other parts of the fuse structure have been deleted for the reasons set forth above in detail in regard to FIGS. 14.

Casing l is provided with a radial bore la having an axial spacing s from the rim 1e of easing 1". The outer surface 1d of easing l is provided with a toroidal groove 1 The radially outer end of bore la is coextensive with the radially inner surface of groove 1] or, in other words, bore la projects into groove 1]. Restraining wire 3" projects from the inside of casing l" radially outwardly into groove 1f, forms one or more turns inside of groove 1] and then projects out of groove If to the outer surface 1d of easing 1". The portion of restraining wire 3" extending beyond groove 1] upon the outer surface 1d of easing l" is clamped against said surface by the lateral surface of ferrule or ferrule means 2" when the latter has been fully mounted on casing 1". No point of restraining wire 3" is clamped between the rim 1e of casing 1 and the inner end surface of ferrule 2".

Basically the structure of FIGS. 5, 6 and 6a operates in the same fashion as that of FIGS. 3 and 4, but the friction established by the turns of the restraining wire 3 formed inside of groove 1] makes it possible to greatly increase the pull exerted upon restraining wire 3" without danger of slippage, or creepage.

Referring now to FIGS. 7 and 8, numeral 1' has been applied to indicate a substantially tubular casing of insulating material corresponding to the part of FIGS. 3 and 4 to which the same reference character has been applied. Casing 1 is preferably a length of tubing of a synthetic resin-glass-cloth laminate. The best material of this kind for the purpose in hand is melamine-glasscloth because it combines mechanical strength with highly desirable electrical characteristics. Casing 1' has a first end or left end and a second end or right end. A first pair of ferrules 2 is mounted on casing 1'. One of ferrules 2' is mounted on the first end or left end of casing 1' closing the latter on the left end thereof. The other of ferrules 2 is mounted on the second or right end of easing 1' closing the latter on the right end thereof. Ferrules 2 are perforated, or provided with a slot, allowing the ends of a fusible element or ribbon fuse link 4 arranged inside of casing 1' to project transversely through the end surfaces of ferrules 2' into pools 20 of solidified solder arranged outside of ferrules 2'. Solderjoints 20 conductively connect the ends of fusible ribbon element 4 to ferrules 2', and thus fusible ribbon element 4 conductively interconnects ferrules 2'. A pulverulent arc-quenching filler 5, preferably quartz sand, fills casing 1' and embeds fusible ribbon element 4. Fusible ribbon element 4 is provided with five points of reduced cross-sectional area, or five necks, of which two points of reduced cross-sectional area, or necks, are sandwiched bet-ween a pair of plates 6 of glass-cloth-melamine. Plates 6 are held in position by means of eyelets 7 projecting transversely through plates 6. The purpose and function of plates 6 are described more in detail in 11.8. Pat. No. 2,964,604 to P.C.Jacobs et al., .Dec. 13,1960 for CURRENT- LIMITING FUSES HAVING COMPOUND ARC- VOLTAGE GENERATING MEANS. Reference may be had to this patent for a more complete understanding of the fuse-link-sandwiching plates 6. A second pair of ferrules 8 is mounted on the ends of casing l. The left ferrule 8 is mounted on left ferrule 2', and the right ferrule 8 is mounted on right ferrule 2. Ferrules 2' and 8 and easing 1 are arranged in coaxial relation. The left ferrule 8 defines a cavity 9 in coaxial relation with casing 1 and housing a spring biased blown fuse indicator generally designated by reference numeral 10. The blown fuse indicator 10 includes a spring housing 11 press-fitted into cavity 9 and housing a helical spring 12. The right end of spring 12 rests against spring housing 1 l and the left end of spring 12 rests against indicating cap 13. Pin 14 is arranged in coaxial relation to cavity 9 and projects through spring housing 11, helical biasing spring 12 and indicating cap 13. The left end of pin 14 is upset in the fashion of a rivet and rests against the end surface of indicating cap 13. The right end of pin 14 is flattened to form a tab-like structure which is perforated. Reference character 3 has been applied to indicate a restraining wire for the blown fuse indicator 10. The left end of restraining wire 3' is threaded through the perforation in the right end of pin 14, and thus firmly affixed to the latter. As seen in FIG. 8, the restraining wire 15 slants downwardly toward the slot la in casing l.

The way in which the right end of restraining wire 3' is firmly held in position against the bias of spring 12 and injury to the right end of restraining wire 3' is precluded by keeping excessive specific pressure away from wire 3 will be apparent from FIGS. 3 and 4 and the context thereof.

The length of restraining wire 3 along which it is clamped against the outer surface of casing l is slightly less than the length of ferrules 2', and for this reason the clamping pressure does not need to be very high. The pull of the spring 12 is mainly taken up by the loop formed by restraining wire 3' and slot la. The axially inner ends of ferrules 2 are rolled into circular grooves lg formed in casing l' and ferrules 8 are mounted upon ferrules 2. Grooves lg are also clearly shown in FIGS.

3 and 4. The outer surfaces of ferrules 2 must be centerless ground after rolling their axially inner edges into grooves lg and before the outer ferrules 8 are mounted on the former. The particular orientation of the slot la facilitates assembly of the fuse, i.e., it facilitates the proper positioning of restraining wire 3' which is necessary in the structure of FIGS. 3, 4 and 8 to preclude severing of restraining wire 3 by the sharp edge of fer rule 2.

It will be apparent from FIGS. 5, 6 and 6a and the context thereof that the fuse structure shown in FIG. 8 may be modified by substituting therein for the restraining wire anchoring means of FIGS. 3 and 4 the restraining wire anchoring means of FIGS. 5, 6 and 6a.

In operation while the fuse is current-carrying, i.e., while ribbon fuse link 4 is current-carrying, virtually no current flows through restraining wire 3' connected in parallel to ribbon fuse link 4 because the resistance of wire 3' is much larger than that of fuse link 4. When the latter is severed by fusion and arcing, a current flows through wire 3' causing melting thereof. As a result indicating cap 13 is propelled by spring 12 out of the left ferrule 8.

I claim as my invention:

1. An electric fuse including a. substantially tubular casing of electric insulating material having a first end and a second end;

a first pair of ferrules, one of said first pair of ferrules being mounted on and closing said first end of said casing and the other of said first pair of ferrules being mounted on and closing said second end of said casing;

c. a fusible element conductively interconnecting said first pair of ferrules;

. a pulverulent arc-quenching filler inside said casing embedding said fusible element;

e. a second pair of ferrules each mounted on one of said first pair of ferrules, one of said second pair of ferrules defining a cavity coaxial with said casing and housing a spring biased blown fuse indicator;

f. a restraining wire for said blown fuse indicator arranged inside said casing and having one end projecting transversely through said one of said first pair of ferrules and being affixed to said blown fuse indicator, said restraining wire having another end clamped in position adjacent the end of said casing remote from said blown fuse indicator; and

g. means for precluding slippage of said restraining wire under the action of the spring bias of said blown fuse indicator, said slippage precluding means including a substantially transverse opening in said casing adjacent the end thereof remote from said blown fuse indicator and a loop formed by said another end of said restraining wire threaded through said opening and being brought out of said casing through said opening to the outer surface of said casing and being clamped by said other of said first pairof ferrules against said outer surface of said casing only at a region thereof spaced from the immediately adjacent rim of said casing.

2. An electric fuse as specified in claim 1 wherein said second end of said casing is provided with a slot slanting from the outer surface of said casing radially inwardly in the direction toward said other of said first pair of ferrules, said slot receiving said restraining wire and spacing said restraining wire from the immediately adjacent rim of said casing.

3. An electric fuse as specified in claim 1 wherein the outer surface of said casing is provided with an annular groove axially spaced from the immediately adjacent rim of said casing at said second end thereof, wherein the radially outer end of said transverse opening is situated inside of said groove, wherein said another end of said restraining wire extends through said transverse opening into said groove and forms turns therein, and wherein a portion of said restraining wire leaving said groove is clamped by said other of said first pair of ferrules against the outer surface of said casing.

4. An electric low voltage fuse including a tubular casing of electric insulating material; a pair of ferrule means for closing the ends of said casing; a pulverulent arc-quenching filler inside of said casing; a fusible element inside said casing submersed in said filler conductively interconnecting said pair of ferrule means; a spring-biased blown fuse indicator arranged adjacent an end surface of one of said pair of ferrule means, and a restraining wire for said blown fuse indicator arranged inside said casing and normally restraining said blown fuse indicator in the non-indicating position thereof, wherein the improvement comprises a radial slot in the end of said casing remote from said blown fuse indicator extending in a direction longitudinally of said casing, the radially inner end of the bottom of said slot being positioned more remotely from the transverse center plane of said casing than the radially outer end of the bottom of said slot so that said bottom of said slot forms an oblique surface, the end of said restraining wire remote from said blown fuse indicator being inserted into said slot and being in engagement with the outer surface of said casing only in a region thereof spaced from the adjacent rim of said casing and being clamped against said region of said outer surface of said casing by the lateral surface of one of said pair of ferrule means, and the portion of said end of said restraining wire inserted into said slot being spaced from the end surface of said one of said pair of ferrule means.

5. An electric low voltage fuse including a tubular casing of electric insulating material; a pair of ferrule means for closing the ends of said casing; a pulverulent arc-quenching filler inside said casing; a fusible element inside said casing submersed in said filler conductively interconnecting said pair of ferrule means; a spring biased blown fuse indicator arranged adjacent an end surface of one of said pair of ferrule means; and a restraining wire for said blown fuse indicator arranged inside said casing and normally restraining said blown fuse indicator in the non-indicating position thereof wherein the improvement comprises a. a coaxial toroidal groove in the outer surface of said casing spaced from the rim of said casing next to said groove;

b. a substantially transverse opening in said casing extending from the inner surface of said easing into said groove;

c. said restraining wire being threaded through said of said groove to the outer surface of said casing opening, forming turns inside said groove and exbeing clamped by one of said pair of ferrule means tending out of said groove to the outer surface of 8 the surface of Sald 335mg at reglo" thereof situated axially inwardly from said groove.

said casing; and d. the portion of said restraining wire extending out 5 

1. An electric fuse including a. substantially tubular casing of electric insulating material having a first end and a second end; B. a first pair of ferrules, one of said first pair of ferrules being mounted on and closing said first end of said casing and the other of said first pair of ferrules being mounted on and closing said second end of said casing; c. a fusible element conductively interconnecting said first pair of ferrules; d. a pulverulent arc-quenching filler inside said casing embedding said fusible element; e. a second pair of ferrules each mounted on one of said first pair of ferrules, one of said second pair of ferrules defining a cavity coaxial with said casing and housing a spring biased blown fuse indicator; f. a restraining wire for said blown fuse indicator arranged inside said casing and having one end projecting transversely through said one of said first pair of ferrules and being affixed to said blown fuse indicator, said restraining wire having another end clamped in position adjacent the end of said casing remote from said blown fuse indicator; and g. means for precluding slippage of said restraining wire under the action of the spring bias of said blown fuse indicator, said slippage precluding means including a substantially transverse opening in said casing adjacent the end thereof remote from said blown fuse indicator and a loop formed by said another end of said restraining wire threaded through said opening and being brought out of said casing through said opening to the outer surface of said casing and being clamped by said other of said first pair of ferrules against said outer surface of said casing only at a region thereof spaced from the immediately adjacent rim of said casing.
 2. An electric fuse as specified in claim 1 wherein said second end of said casing is provided with a slot slanting from the outer surface of said casing radially inwardly in the direction toward said other of said first pair of ferrules, said slot receiving said restraining wire and spacing said restraining wire from the immediately adjacent rim of said casing.
 3. An electric fuse as specified in claim 1 wherein the outer surface of said casing is provided with an annular groove axially spaced from the immediately adjacent rim of said casing at said second end thereof, wherein the radially outer end of said transverse opening is situated inside of said groove, wherein said another end of said restraining wire extends through said transverse opening into said groove and forms turns therein, and wherein a portion of said restraining wire leaving said groove is clamped by said other of said first pair of ferrules against the outer surface of said casing.
 4. An electric low voltage fuse including a tubular casing of electric insulating material; a pair of ferrule means for closing the ends of said casing; a pulverulent arc-quenching filler inside of said casing; a fusible element inside said casing submersed in said filler conductively interconnecting said pair of ferrule means; a spring-biased blown fuse indicator arranged adjacent an end surface of one of said pair of ferrule means, and a restraining wire for said blown fuse indicator arranged inside said casing and normally restraining said blown fuse indicator in the non-indicating position thereof, wherein the improvement comprises a radial slot in the end of said casing remote from said blown fuse indicator extending in a direction longitudinally of said casing, the radially inner end of the bottom of said slot being positioned more remotely from the transverse center plane of said casing than the radially outer end of the bottom of said slot so that said bottom of said slot forms an oblique surface, the end of said restraining wire remote from said blown fuse indicator being inserted into said slot and being in engagement with the outer surface of said casing only in a region thereof spaced from the adjacent rim of said casing and being clamped against said region of said outer surface of said casing by the lateral surface of one of said pair of ferrule means, and the portion of said end of said restraining wire inserted into said slot being spaced from the end surface of said one of said pair of ferrule means.
 5. An electric low voltage fuse including a tubular casing of electric insulating material; a pair of ferrule means for closing the ends of said casing; a pulverulent arc-quenching filler inside said casing; a fusible element inside said casing submersed in said filler conductively interconnecting said pair of ferrule means; a spring biased blown fuse indicator arranged adjacent an end surface of one of said pair of ferrule means; and a restraining wire for said blown fuse indicator arranged inside said casing and normally restraining said blown fuse indicator in the non-indicating position thereof wherein the improvement comprises a. a coaxial toroidal groove in the outer surface of said casing spaced from the rim of said casing next to said groove; b. a substantially transverse opening in said casing extending from the inner surface of said casing into said groove; c. said restraining wire being threaded through said opening, forming turns inside said groove and extending out of said groove to the outer surface of said casing; and d. the portion of said restraining wire extending out of said groove to the outer surface of said casing being clamped by one of said pair of ferrule means against the outer surface of said casing at a region thereof situated axially inwardly from said groove. 